Pogonophora: Their spectacular role in animal evolution
A spectacular
connection of the two main higher animal groups
Early embryology of the two main lines of animals
The Pogonophora
show how the radial and indeterminate cleavage of deuterostomes (echinoderms, hemichordates
and vertebrates) came about from pre-Cambrian annelids in the deep sea while
other annelids retained the spiral and determinate cleavage of protostomes as
they were giving rise to arthropods and mollusks.
The deep sea had
low input of nutrients that put a premium on not investing in heavy egg shells
that protected eggs of protostomes as they were confined in the spiral pattern
of cleavage, a confinement that was released by less confining outer membranes
that allowed a more direct radial cleavage pattern. At the same time radial cleavage did not
produce the immediate cell fate into a particular tissue, thus making possible
more than one viable embryo from a single egg as each of the early cleavage
cells retained all of the development potential of the original egg. The ability to complete development without
all the original cleavage products had high survival value in the rigorous deep-sea
environment, although details are speculative.
My comparative
study of Tasmanian and Michigan isopods included observation of the impact of the
egg membrane’s role in confining development, leading to a unique egg
appendage, a clear indication that evolution can occur in developmental stages
independent of adult development. (see
more on “Origin of deuterostome embryology” in blog post dated 6/24/2013)
Systems inversion
A simple process
The inverted position of blood vessels, nerves, oral
openings of deuterostomes as compared to protostomes (especially the ancestral
annelids of both groups) was proposed as evidence of the central role of
annelids in evolution of higher animal phyla.
The original reason for rejecting the theory was the drastic difference
in early embryology of the two lines of animals. The next reason thought to negate the annelid
theory as well as the embryological argument against it was the use of
nucleotide and other molecular data.
Such data must be realigned taking into consideration the effect of the
astronomically slow rate of genetic change in the pogonophorans ancestors.
Inversion, the first step
The inversion of annelids began with certain polychaetes
that began living vertically in tubes they secreted, as seen in Sabella and many other shallow water
polychaetes. As those growing in
progressively deeper water, with less food, became dependent on absorption of
nutrients in pore water of the sediments they outcompeted those wasting energy
on producing a mouth and some bilateral structure. This stage is still found in abyssal
pogonophorans.
Inversion, the second step
The return of descendants to shallow seas occurred once the
worst episode of pre-Cambrian asteroid bombardment eased. As they arrived in shallower more nutrient
rich areas, they reformed the remnants of their digestive system with a new
mouth on the former dorsal side which became the ventral side, as they groped
around the sediment surface near their tube, finding food particles that pushed
the epidermal and gut layer together triggering mouth formation on the former
dorsal surface without the restriction of the nervous system that originally had encircled the esophagus. Other
clues to this step are presented by the parallels of endocrine hormone
function, transport, and structural similarities of vertebrates, arthropods,
and annelids.
Protostome-Deuterostome
links
Segmentation/Metamerism
The segmentation of annelid type was found on a short portion of
the most deeply embedded part of some pogonophorans; it included setae that are
a very annelid like characteristic. A
few anterior regions are noticeable but without the posterior segmented region it would be difficult to make an annelid connection. The metamerism of chordates such as ourselves
seen in bone, muscle, nervous system and blood vessels is now easily
understandable with the intermediate stage of pogonophorans.
The transition from pogonophorans to chordates is best shown
by the larval stage comparisons of pogonophorans and hemichordates.
Molecular evidence
Molecular features of several types show greater similarity
between deuterostomes and advanced protostomes than their earliest variants
found in more ancient protostomes once thought to be the closest common
ancestors at the protostome-deuterostome split.
DNA/RNA studies of evolutionary relationships at the phylum
level need reevaluation because major ones have ignored the mutation rate
differences associated with generation times.
Many well focused studies have shown generation time does affect
evolution rates. One impact has been the
Pogonophora showing up in many odd places in phylogenetic trees because they
are almost unchanged since their divergence from major groups that have
diverged even more from more recent relatives.
Six other posts, from June 17 to June 20, 2013 have
additional clarification of the points made above. The second June 30th post of that year
is an annotated bibliography that has some emphasis on protostome/deuterostome
comparisons.
Joseph Engemann
Emeritus Professor of Biology, Western Michigan University, Kalamazoo,
Michigan May 19, 2019
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